Long fiber thermoplastic process for conductive composites and composites formed thereby

ABSTRACT

The present invention relates to a polymer article that includes electrically conductive fibers to provide electrical electromagnetic interference (EMI) shielding and their method of manufacture. The invention includes a method for forming shielding materials by impregnating conductive fibers in a polymer material via direct injection of the conductive fibers into the extrusion process. The invention also includes EMI shielding polymers and products that are radio-frequency and electromagnetically shielded by parts formed of the shielding polymer.

This Application claims the benefit of U.S. Provisional Application60/729,695, filed Oct. 24, 2005.

TECHNICAL FIELD AND INDUSTRIAL APPLICABILITY OF THE INVENTION

The present invention relates to a polymer article that includeselectrically conductive fibers to provide electrical electromagneticinterference (EMI) shielding and their method of manufacture. Moreparticularly, the invention relates to a method for forming shieldingmaterials by impregnating conductive fibers in a polymer material viadirect injection of the conductive fibers into the extrusion process.EMI shielded polymers of the present invention may be formed into a widevariety of products such as radio-frequency and electromagnetic shieldedplastic articles.

BACKGROUND OF THE INVENTION

With the increased usage of electronic equipment such as computers andother digital devices there is a heightened concern for the hazardsassociated with electromagnetic radiation, in particular radar waves,microwaves and electromagnetic radiation produced by electroniccircuits. As the electronic industry continues to grow at a rapid pace,there exists a need to create improved electromagnetic wave shieldingmaterials, which can be incorporated into electronic products.

A number of electrically conductive materials have been developed tofabricate composite articles, such as plastic articles, forelectromagnetic shielding, electrostatic dissipation, and otherelectrically enhanced characteristics. Plastic articles formed fromelectrically conductive materials are particularly convenient ascompared to traditional metal materials because they are lightweight,easily produced using injection molding techniques, and low cost.Typically these electrically conductive materials are composites ofplastics and conductive powders and chopped fibers.

Various techniques have been employed when incorporating electricallyconductive powders and chopped fibers into a composite article. FIG. 1illustrates a traditional thermoplastic extrusion compounding technique,which has been commonly employed. A thermoplastic resin 112 is fed intothe compounder 110. The resin 112 is heated to a molten temperature andthen fibers or powders (collectively indicated as 114) are fed into thecompounder 110 to mix in the conductive powders or chopped fibers. Theresin/broken fiber mixture is extruded 118, cooled in a water bath 120,then chopped by a Strand Cutter 122 into pellets 124. Pellets 124 arethen typically fed into the melting section of an injection moldingmachine (not shown).

The pellets of the process shown in FIG. 1, include fibers that arebroken due to the cutting action by the screw 116 and by the shear forceapplied to melt the resin. The fibers are broken during the compoundingprocess so that the resulting composite article contains only relativelyshort broken fibers. The shortened fibers impart reduced electromagneticshielding properties to the composite due to their reduced ability toform a conductive fiber network and conduct electricity through thecomposite article. Alternatively, when mixing conductive powders withthe molten thermoplastic it is typically necessary to employ a verylarge amount of the conductive powder. Such large amounts of powder canresult in a poor dispersion of the powder or reduced mechanical strengthof the final product. Accordingly, composite articles formed with brokenfibers and powders require higher loadings or filler concentrationswhich leads to decreased mechanical strength of the composite articleformed and higher material costs.

Published U.S. Application US 2002/0108699 entitled “Method for FormingElectrically Conductive Fibers and Fiber Pellets” (herein incorporatedin its entirety by reference) discloses electromagnetic wave shieldingpellets formed by (1) applying a sizing material to Ni-coated fibers tomake the fibers compatible with the thermoplastic matrix material; (2)thermally drying the sizing; (3) wire coating the fibers with thethermoplastic matrix material; (4) quenching the thermoplastic matrixmaterial; (5) drying the coated fibers; (6) chopping or pelletizing thecoated fibers; (7) feeding the pellets into an injection moldingmachine; and (8) melting the pellets and injection molding the part.

Each step of the pelletized fiber approach allows for material loss andinefficiency, slows cycle times and provides an opportunity for defects.There are several additional drawbacks. Heating of the thermoplasticpolymer during the wire coating process and again during the injectionmolding process degrades the performance of the polymer. Severedegradation can break down the polymer and form gasses that result invoids and a subsequent loss of shielding and mechanical properties.Additionally, to achieve good mold filling, the polymer must be at amelt flow sufficient to fill ribs or other small features of a part.Melt flow is achieved by the selection of the thermoplastic material,the temperature, dwell time and shear from the screw. High shear fromthe screw provides for sufficiently high melt flow but breaks theconductive fibers into smaller and smaller lengths and diminishes theability of the fibers to form a continuous network of fibers.

Shielding effectiveness may be determined by ASTM-D4935, which measuresfar field shielding effectiveness, or by ASTM ES7-83, which measuresnear field effectiveness.

US 2002/0108699 produces a electromagnetic shielded article with ashielding effectiveness of 80-90 dB (far field) and less than 80 dB(near field) at a frequency of 30-1500 MHz and a 15 wt. % fiber loading.

An alternative dry blend method of forming shielded articles requiresthat chopped conductive fibers are mixed with the resin directly at theinjection molding operation. This typically results in very poor fiberdispersion and inconsistent electrical performance from part to part. US2002/0108699 discloses that the dry blend method produces aelectromagnetic shielded article with a shielding effectiveness of 60-70dB at a frequency of 30-1500 MHz and a 15 wt. % fiber loading.

FIG. 1 shows the in-line process according to US 2002/0108699 in which afiber tow 103 is unwound from a package or spool 105 and drawn throughan aqueous silane bath 106 to apply a conductive coating to the tow 103.The tow 103 is then drawn through an aqueous silane bath 104 and throughan oven 108. The tow 103 then passes though a nonaqueous sizing bath107. The tow 103 is then wound onto a package (or spool) 113. The coatedfibers are subsequently pelletized and place into the extruder of aninjection-molding machine.

Previous methods of forming EMI shielding composite articles have notbeen entirely satisfactory for due to short fiber length in the finishedpart which reduces the shielding and necessitates additional loading offibers. The average fiber length from wire coating, pelletizing andsubsequent injection molding the pellets was about 0.5 mm. Because theconductive fibers must touch one another to form a continuous networkand provide shielding, 10-20 wt. percent of carbon fiber or nickelcoated carbon fiber is required to provide sufficient EMI shielding.This high level of fiber loading increases the cost of the compositearticle due to the high cost of the fibers and inhibits the flow of thepolymer in the mold. The high fiber loading also significantly increasesthe modulus of the article but lowers impact resistance. EMI shieldingresins are used in goods such as mobile phones and lap top computerswhich are expected to resist the impact from a fall of up to a meter ormore without breakage. A thermoplastic resin filled to 10-20 wt. %fibers becomes brittle and susceptible to breakage.

Another drawback to the previous methods of forming EMI shieldingcomposite articles is poor fiber dispersion. The pelletized material isdifficult to process such that the fibers are fully dispersed and do notform an efficient network of conducting fibers in the compositematerial. When good fiber distribution is sometime achieved; however,the pellets and fibers will have been comminuted to such an extend thatthe average fiber length is approximately 0.5 mm and, as a result, theshort fibers do not form an efficient network of conducting fibers inthe composite material. In either case, it is necessary to overload thecomposite with conducting fibers to compensate for the inefficientnetwork of conducting fibers in the composite material.

U.S. Pat. No. 6,676,864 entitled “Resin and Fiber Compounding Processfor Molding Operations” (herein incorporated in its entirety byreference) describes an apparatus and process for preparing fiberreinforced resin and molding that resin. The '864 patent shows aninjection molding apparatus including a two stage extruder the first toimpart shear forces to melt a polymer and the second to feed moltenthermoplastic into a mold. Reinforcing fibers, such as glass fibers,carbon-graphite fiber or Kevlar fibers, are supplied between the twostages of the extruder. The molding device of U.S. Pat. No. 6,676,864does not contemplate electromagnetic shielding.

SUMMARY OF THE INVENTION

The invention answers the problems connected with previous methods offorming EMI shielded polymer. Long fiber thermoplastic technology allowsthe conductive fibers, to maintain a length sufficient to provide EMIshielding at lower fiber loading. The long fiber thermoplastic processfor forming EMI shielding composite articles also provides increasedimpact resistance, enhanced surface aesthetics and improved extrusionand injection molding processing at a lower material cost and withdecreased waste and scrap.

The long fiber thermoplastic process for conductive composites andcomposites formed thereby of the present invention is simpler, moreefficient and provides improved properties than the prior art methods.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the principle of theinvention. In the drawings:

FIG. 1 is a schematic representation of a prior art wire coating processfor compounding an EMI shielding thermoplastic extrusion material.

FIG. 2 is a plan view, partially in cross section, of one extruderuseful in practicing the present invention.

DETAILED DESCRIPTION AND PREFERRED EMBODIMENTS OF THE INVENTION

Thermoplastic resin, preferably in the form of pellets, is provided toresin primary extruder 12 from a resin supply 14. The resin may be anyof a variety of acceptable thermoplastic resins for the product purposeintended, such as polypropylene, nylon, polyurethane, and polyesters. Amelting screw 16 is rotates within melting barrel 18 of extruder 10.While the shear force of melting screw 16 may provide sufficient heatingto melt and condition the polymer, the melting barrel 18 may be providedwith an additional heat source as is known in the art.

A flow control plate 20 may be used at the downstream end of barrel 40to control the flow of resin 15 out of the extruder barrel 18 and intocoating die 22. The plate 20 typically restricts the flow of resin 15 bya reduction in the diameter or by otherwise constricting the flow withinthe barrel 18. The coating die 22 and any apparatus in contact with theresin 15 may include suitable heat elements to maintain the desiredtemperature of the resin. The pressure within the coating die may bemonitored by a pressure transducer which provides a control signal tothe drive motor 17 of melting screw 16.

Fiber spool 24 provides a direct feed of a tow of shielding fibers 26.The shielding fibers may be of any suitable composition for examplenickel, copper or and conductive material coated on carbon, aramid,glass or other suitable substrate, alternatively stainless steel, copperor similar metallic fibers may be used. The fibers are pulled thoughinjection nozzle 28 into the coating chamber 32 of coating die 22. Theshielding fibers 26 are then intimately blended and coated with themolten polymer material 15. The coated shielding fibers 26 then exit thecoating die 22 through die orifice 36 of interchangeable insert 30. Thediameter of the die orifice 36 can adjusted by changing insert 30 tocontrol the ratio of shielding fibers 26 to resin 15.

The resin 15, fiber 26 mixture exits coating die 22 and the shieldingfibers 26 may be cut by blade 52 in cutting chamber 50 in housing 54,56. The mixture of resin 15 and fibers 26 exit chamber 50 via orifice 58into extruder 60. The extruder 60 typically includes a barrel 62 thatfeeds the mixture of resin 15 and fibers 26 into extrusion die 64. Afeed screw 66 rotates with barrel 62 and may optionally reciprocatealong axis 72 to feed a charge of molding material through orifice 63into the molding cavity 68 of die 64. The feed screw 66 is driven by apower unit 70.

The temperature within the barrel 18, coating chamber 32, cuttingchamber 50 and extruder 60 may be controlled by one or more heatingelements and temperature probes controlled a microprocessor (not shown).Suitable polymers include thermoplastic polymers such asacrylonitrile-butadiene-styrene (ABS), polyethylene terephthalate (PET),polybutylene terephthalate (PBT), polyethylene (PA) and otherthermoplastic materials have suitable mechanical, thermal and melt flowproperties.

Other fibers include metal coated glass and carbon fibers includingcoatings of metals such as aluminum, copper, nickel, and lead. Plasmadeposition, molten liquid deposition and electrodepositing of the metalsare preferred methods of coating the fibers.

The article molded within molding cavity 68 includes a number ofindividual shielding fibers 26 where each fiber is of a predeterminedlength, as established by the action of blade 52. The molded articleincludes fibers which are roughly 3 times the length of wire-coated andpelletized method shown in FIG. 1. Average lengths from wire coating,pelletizing and injection molding the pellets was about 0.5 mm. Expectedaverage lengths of the Ni—C fiber would be 1.5 mm or more.

The shielding properties of the molded material are defined by thenumber of fibers in contact to create a conductive network. The longfiber thermoplastic should be able to reduce the number of fibers neededto achieve the same connectivity to ¼ to ⅓ the original amount.

Thus, rather than needing 15%-20% by weight fiber loading, LFTP couldreduce the fiber needed to as low as 4-5% by weight of composite.

Reducing the amount of carbon fiber to say 5% loading would vastlyimprove the impact strength. The loss of impact strength severely limitsthe number of applications for conductive composites: e.g. cell phoneexteriors. We could anticipate up to 50% impact improvement or morereducing fiber volumes to 5% by weight.

5% fiber loading would also improve the surface aesthetics, anotherissue with exterior housings for consumer products. Fibers at thesurface give a poor appearance.

5% fiber loading would save up to 75% of the fiber costs, which can cost$30/lb or more.

EXAMPLES

The following examples are prophetic and all mechanical and shieldingproperties are estimated. In Example 1, acrylonitrile-butadiene-styrene(ABS) polymer is melted and 10% by weight Nickel coated carbon (NCC)fibers are added to the ABS. The fibers are cut to length and theABS/fiber melt is extruded to form a composite part.

In Example 2, ABS polymer is melted and 7.5% by weight NCC fibers areadded to the ABS. The fibers are cut to length and the ABS/fiber melt isextruded to form a composite part.

In Example 3, ABS polymer is melted and 5.0% by weight NCC fibers areadded to the ABS. The fibers are cut to length and the ABS/fiber melt isextruded to form a composite part. The properties of the examples areestimated below. EXAMPLE WT % NCC Impact 1 10 Pass 2 7.5 Pass 3 5.0 Pass

The invention of this application has been described above bothgenerically and with regard to specific embodiments. Although theinvention has been set forth in what is believed to be the preferredembodiments, a wide variety of alternatives known to those of skill inthe art can be selected within the generic disclosure. The invention isnot otherwise limited, except for the recitation of the claims set forthbelow.

1. A fiber reinforced polymer article having improved electromagneticshielding properties, comprising: a thermoplastic polymer; and less than10 percent by weight conducting fibers wherein the composite article hasan electromagnetic shielding efficiency of at least 70 dB.
 2. The fiberreinforce polymer article of claim 1, wherein the electromagneticshielding efficiency is least 90 dB.
 3. The fiber reinforce polymerarticle of claim 1, wherein said conducting fibers are present in anamount less than 5 percent by weight
 4. The fiber reinforce polymerarticle of claim 1, wherein said thermoplastic is selected from thegroup consisting of acrylonitrile-butadiene-styrene (ABS), polyethyleneterephthalate (PET), polybutylene terephthalate (PBT) and polyethylene(PA).
 5. The fiber reinforce polymer article of claim 1, whereinproperty deterioration of said thermoplastic due to melt history issubstantially reduced
 6. A method of manufacturing a composite articleincluding the steps of: melting a thermoplastic material; addingconductive fibers to said melted thermoplastic material; cutting saidconducting fibers to a predetermined length; and injection molding thethermoplastic material to form a composite article having aelectromagnetic shielding efficiency of at least 70 dB.
 7. A digitaldevice having an electromagnet shield, said shield comprising: athermoplastic polymer; and less than 10 percent by weight conductingfibers wherein the composite article has an electromagnetic shieldingefficiency of at least 70 dB.
 8. The fiber reinforce polymer article ofclaim 7, wherein the electromagnetic shielding efficiency is least 90dB.